Ultra-wideband is a radio technology that transmits digital data across a very wide frequency range, 3.1 to 10.6 GHz. It makes use of ultra low transmission power, typically less than −41 dBm/MHz, so that the technology can literally hide under other transmission frequencies such as existing Wi-Fi, GSM and Bluetooth. This means that ultra-wideband can co-exist with other radio frequency technologies. However, this has the limitation of limiting communication to distances of typically 5 to 20 metres.
There are two approaches to UWB: the time-domain approach, which constructs a signal from pulse waveforms with UWB properties, and a frequency-domain modulation approach using conventional FFT-based Orthogonal Frequency Division Multiplexing (OFDM) over Multiple (frequency) Bands, giving MB-OFDM. Both UWB approaches give rise to spectral components covering a very wide bandwidth in the frequency spectrum, hence the term ultra-wideband, whereby the bandwidth occupies more than 20 percent of the centre frequency, typically at least 500 MHz.
These properties of ultra-wideband, coupled with the very wide bandwidth, mean that UWB is an ideal technology for providing high-speed wireless communication in the home or office environment, whereby the communicating devices are within a range of 20 m of one another.
FIG. 1 shows the arrangement of frequency bands in a Multi Band Orthogonal Frequency Division Multiplexing (MB-OFDM) system for ultra-wideband communication. The MB-OFDM system comprises fourteen sub-bands of 528 MHz each, and uses frequency hopping every 312 ns between sub-bands as an access method. Within each sub-band OFDM and QPSK or DCM coding is employed to transmit data. It is noted that the sub-band around 5 GHz, currently 5.1-5.8 GHz, is left blank to avoid interference with existing narrowband systems, for example 802.11a WLAN systems, security agency communication systems, or the aviation industry.
The fourteen sub-bands are organized into five band groups, four having three 528 MHz sub-bands, and one band group having two 528 MHz sub-bands. As shown in FIG. 1, the first band group comprises sub-band 1, sub-band 2 and sub-band 3. An example UWB system will employ frequency hopping between sub-bands of a band group, such that a first data symbol is transmitted in a first 312.5 ns duration time interval in a first frequency sub-band of a band group, a second data symbol is transmitted in a second 312.5 ns duration time interval in a second frequency sub-band of a band group, and a third data symbol is transmitted in a third 312.5 ns duration time interval in a third frequency sub-band of the band group. Therefore, during each time interval a data symbol is transmitted in a respective sub-band having a bandwidth of 528 MHz, for example sub-band 2 having a 528 MHz baseband signal centred at 3960 MHz.
The technical properties of ultra-wideband mean that it is being deployed for applications in the field of data communications. For example, a wide variety of applications exist that focus on cable replacement in the following environments:                communication between PCs and peripherals, i.e. external devices such as hard disc drives, CD writers, printers, scanner, etc.        home entertainment, such as televisions and devices that connect by wireless means, wireless speakers, etc.        communication between handheld devices and PCs, for example mobile phones and PDAs, digital cameras and MP3 players, etc.        
The antenna arrangements used in ultra-wideband systems are usually omni-directional, meaning that radio signals are emitted in all directions from an active radiating element, or elements. However, it is desirable to be able to alter the profile of the emitted radio signals so that they are emitted from the antenna arrangement in a particular direction or directions. In addition, it is desirable to be able to switch an antenna arrangement with more than one active radiating element from an omni-directional mode to a mode in which the antenna arrangement serves a number of different sectors.
By directing the emitted radio signals in a particular direction or directions, interference with other nearby communication links can be reduced, thereby allowing the capacity of the communication system (in terms of the number of possible communication links) to be increased.
Although fixed beam directional antennas are known, for example, horns, reflector or planar linear and conformal arrays based on a plurality of active radiating elements each of which is individually fed and appropriately phased these fixed conventional arrangements can only provide a limited range of coverage with the directed beam. Furthermore, in these conventional arrangements the direction of the beam cannot be switched particularly quickly. A number of directional beam technologies suffer from the limitation that the width of the main peak of the radiated beam depends on the wavelength of the radio signals emitted. Phased arrays based on a plurality of individually fed (with tailored distribution in amplitude and phase) active elements can in principle provide adjustable beams in shape and angular position. However, these antennas are unacceptably expensive. In addition, the state of the art of these antennas suggest that these structures will be less capable of covering the UWB bandwidth, primarily due to mutual coupling or grating lobe problems. Thus, these conventional antenna arrangements are not particularly suitable for use in ultra-wideband systems intended for consumer electronic applications.
It is therefore an object of the invention to provide a directional antenna arrangement for use in an ultra-wideband system that overcomes the problems with the above conventional systems.